1) Could you please help me with this?
2) Can you please help me with this?
Which one is correct? I've heard people use the first one a lot but why should you use Past Tense when asking someone to do something?
It's not past tense in that case. It's just a polite way of making a request.
With the modals, past tense does not mean past time necessarily. We use it to create a distance (social) between ourselves and the person we are talking to. This is probably because we don'y have two, or more, forms for the second person pronouns.